Permanent magnets based on compositions containing iron, neodymium and/or praseodymium, and boron are known and in commercial usage. Such permanent magnets contain, as an essential magnetic phase, grains of tetragonal crystals in which the proportions of, for example, iron, neodymium and boron are exemplified by the empirical formula Nd.sub.2 Fe.sub.14 B. These magnet compositions and methods for making them are described by Croat in U.S. Pat. No. 4,802,931 issued Feb. 7, 1989. The grains of the magnetic phase are surrounded by a second phase that is typically rare earth-rich, as an example neodymium-rich, as compared with the essential magnetic phase. It is known that permanent magnets based on such compositions may be prepared by rapidly solidifying, such as by melt spinning, a melt of the composition to produce fine grained, magnetically isotropic platelets of ribbon-like fragments. Magnetically isotropic magnets may be formed from these isotropic particles by practices which are known, such as by bonding the particles together with a suitable resin.
To improve magnetic properties, it is known to hot press the isotropic particles to form an isotropic magnetic body and then hot work the isotropic magnetic body to create high strength, magnetically anisotropic permanent magnets, as taught by U.S. Pat. No. 4,782,367 to Lee. Being magnetically anisotropic, such magnets exhibit excellent magnetic properties, typically having high remanence and a magnetic coercivity of about one kiloOersted (kOe) or higher.
However, a shortcoming of such anisotropic magnets is that, because the final forming step is a hot working operation, the shapes in which the anisotropic magnets can be formed are significantly limited, particularly in comparison to the great variety of shapes which are possible with bonded magnets. Furthermore, even if an uncomplicated shape is required for a particular application, it is generally economically undesirable to fabricate a special punch and die for each application which arises, particularly when the magnet is intended for evaluation in a development program, as opposed to a full scale production program. Such an approach is also undesirable from the standpoint of lead time, particularly when there is an immediate requirement in a development program for a permanent magnet whose size is currently unavailable.
As a result, it is a common practice in the art to form large anisotropic magnets and then machine these magnets to form smaller magnet bodies which are sized and shaped to meet the requirements of a particular application. Many methods are known for machining the smaller magnet bodies from the larger anisotropic magnets, with electrical discharge machining (EDM) being the most common technique, although other methods such as using a diamond cutting wheel or other appropriate material are also employed to machine the magnets. However, because such anisotropic magnets are formed by a hot working operation, these magnets are generally characterized as being hard and brittle. As a result, it is a conventional practice to anneal these magnets prior to machining to enhance their machinability. Typically, annealing is conducted at a temperature of about 350.degree. C. for a duration of at least about eight hours. While this operation successfully serves to promote the machinability of the magnets, an undesirable loss in magnetic properties occurs.
FIG. 1 illustrates the degree to which the magnetic properties of a magnetically anisotropic magnet may be reduced when the above annealing operation is employed. Curve A represents the initial demagnetization curve for an anisotropic permanent magnet having a composition, on a weight percent basis, of about 30.5 percent rare earth, about 2.5 percent cobalt, about 1 percent boron, with the balance being essentially iron. The x-axis represents intrinsic coercivity in kilo-oersteds (kOe), the y-axis represents remanence in kilogauss (kG), and the series of parallel curves numbered 20 through 36 demarcate the energy product (BHmax) in megagauss-oersteds (MGOe). Prior to annealing, the remanence, or residual induction, (B.sub.r) of the magnet was about 12.7 kG, while its intrinsic coercivity (H.sub.ci) was about 13.3 kOe. Curve B represents the demagnetization curve for the permanent magnet after having been annealed at about 350.degree. C. for about eight hours. While remanence remained essentially the same, a significant reduction in intrinsic coercivity occurred, from about 13.3 kOe to about 10.6 kOe. Though such lower intrinsic coercivities are sufficiently high for numerous applications, more demanding applications often cannot be fulfilled with a permanent magnet having an intrinsic coercivity at this level.
From the above, it can be seen that there does not currently exist a suitable method by which a magnetically anisotropic magnet can be readily machined to fulfill a specific application without the magnet produced exhibiting magnetic properties which have been significantly reduced from that of the parent magnet. Therefore, what is needed is a method by which a permanent magnet can be fabricated from a larger permanent magnet without a significant loss in magnetic properties occurring, wherein the method is compatible with conventional machining techniques by which smaller, more intricately shaped magnets can be fabricated from larger, hot worked magnets.